Do not drop sys_boot

This fixes #2391
Add warning about SYS_BOOT capability with pre-3.4 kernels and pre-0.8 LXC.
Merge pull request #1 from jpetazzo/2392-add-kernel-docs

Add warning about SYS_BOOT capability with pre-3.4 kernels and pre-0.8 LXC.
Merge pull request #2712 from makinacorpus/master

Do not drop sys_boot